developed by expressing ATF1 gene from S. cerevisiae which converts isoamyl
alcohol in the growth media to isoamyl acetate. The gene was cloned between a
bacterial ribosome binding site and a transcriptional terminator to make a three part
‘banana-odour generator’. A bacterial promoter that is primarily active during
stationary phase controls the expression of ATF1 gene. For the best production of
the banana fragrance, the cellular chasis bore a mutation in the tnaA gene, which
inhibits indole production and thus effectively eliminates the putrid smell that
characterizes the usual smell of E. coli culture (Dixon and Kuldell 2011).
3-methylbutyl-acetate, a fruity ester, has been synthesized de novo by the yeast
Williopsis saturnus. The yield was improved by feeding fusel oil as a cheap source of
precursor branched alcohols in the fermentation process (Vandamme 2003).
Geotrichum klebahnii has been reported to produce de novo various ethyl esters of
branched carboxylic acids, giving a pleasant fruity flavour. Ethyl-2-methylbutyrate
is formed when medium is supplemented with isoleucine (Janssens et al. 1989).
Another ester, ethyl acetate is responsible for an ‘apple’ aroma. Lai et al. (2019)
reported the production of ethyl acetate in ‘Kaoliang’, a wheat-based koji prepared
using sorghum as a substrate, by S. cerevisiae as the dominant strain along with other
yeasts Kazachstania exigua and Candida humilis.
7.3.2.4 Ketones
The most important flavoured ketone is Diacetyl (2, 3-butanedione) which is responsible for the buttery aroma of many dairy products. Lactic acid bacteria, especially
L. lactis biovar, produces diacetyl from co-fermentation of citrate and lactose
(Papagianni et al. 2007). The characteristic aroma of raspberry is because of parahydroxyphenyl-butan-2-one. The first biotechnological strategy for the production
of raspberry ketone involves the de novo synthesis by basidiomycetes Nidula niveotomentosa using L-tyrosine or L-phenylalanine as the natural precursor. The second
strategy is a two-step bioconversion which involves hydrolysis of betuloside, a
glycoside of 4-4-(hydroxyphenyl)-2-butanol. The hydrolysis releases betuligenol
which gets transformed to ketone by Acetobacter aceti (Schrader 2007).
7.3.2.5 Fruity Lactones
Lactones are the compounds responsible for flavours like oily-peachy, creamy,
fruity, nutty and coconut. Specifically, γ- and δ-lactones with equal or less 12 carbons
are well-known for their great variety of taste and aroma (Dabbou et al. 2016).
γ-decalactone and δ-decalactone are responsible for typical flavour of peach and
apricot (Greger and Schieberle 2007). A yeast Sporobolomyces odorus de novo
produces 4-decalactone (up to 1.6 mg.L) which gives a peach odour (Welsh et al.
1989). Another lactone which is widely used for its coconut odour is 6-pentyl-2pyrone which is produced by fungus Trichoderma viride (Fadel et al. 2015).
Tyromyces sambuceus and Cladosporium suaveolens efficiently generate coconutflavoured lactones γ-decalactone and δ-dodeca-lactone from ricinoleic acid and
linoleic acid, respectively (Kapfer et al. 1989; Allegrone et al. 1991). The yield of
lactones carried by yeasts via de novo fermentation is low, which can be increased
by supplying a limiting intermediate or precursor molecule to the fungal culture.
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alcohol in the growth media to isoamyl acetate. The gene was cloned between a
bacterial ribosome binding site and a transcriptional terminator to make a three part
‘banana-odour generator’. A bacterial promoter that is primarily active during
stationary phase controls the expression of ATF1 gene. For the best production of
the banana fragrance, the cellular chasis bore a mutation in the tnaA gene, which
inhibits indole production and thus effectively eliminates the putrid smell that
characterizes the usual smell of E. coli culture (Dixon and Kuldell 2011).
3-methylbutyl-acetate, a fruity ester, has been synthesized de novo by the yeast
Williopsis saturnus. The yield was improved by feeding fusel oil as a cheap source of
precursor branched alcohols in the fermentation process (Vandamme 2003).
Geotrichum klebahnii has been reported to produce de novo various ethyl esters of
branched carboxylic acids, giving a pleasant fruity flavour. Ethyl-2-methylbutyrate
is formed when medium is supplemented with isoleucine (Janssens et al. 1989).
Another ester, ethyl acetate is responsible for an ‘apple’ aroma. Lai et al. (2019)
reported the production of ethyl acetate in ‘Kaoliang’, a wheat-based koji prepared
using sorghum as a substrate, by S. cerevisiae as the dominant strain along with other
yeasts Kazachstania exigua and Candida humilis.
7.3.2.4 Ketones
The most important flavoured ketone is Diacetyl (2, 3-butanedione) which is responsible for the buttery aroma of many dairy products. Lactic acid bacteria, especially
L. lactis biovar, produces diacetyl from co-fermentation of citrate and lactose
(Papagianni et al. 2007). The characteristic aroma of raspberry is because of parahydroxyphenyl-butan-2-one. The first biotechnological strategy for the production
of raspberry ketone involves the de novo synthesis by basidiomycetes Nidula niveotomentosa using L-tyrosine or L-phenylalanine as the natural precursor. The second
strategy is a two-step bioconversion which involves hydrolysis of betuloside, a
glycoside of 4-4-(hydroxyphenyl)-2-butanol. The hydrolysis releases betuligenol
which gets transformed to ketone by Acetobacter aceti (Schrader 2007).
7.3.2.5 Fruity Lactones
Lactones are the compounds responsible for flavours like oily-peachy, creamy,
fruity, nutty and coconut. Specifically, γ- and δ-lactones with equal or less 12 carbons
are well-known for their great variety of taste and aroma (Dabbou et al. 2016).
γ-decalactone and δ-decalactone are responsible for typical flavour of peach and
apricot (Greger and Schieberle 2007). A yeast Sporobolomyces odorus de novo
produces 4-decalactone (up to 1.6 mg.L) which gives a peach odour (Welsh et al.
1989). Another lactone which is widely used for its coconut odour is 6-pentyl-2pyrone which is produced by fungus Trichoderma viride (Fadel et al. 2015).
Tyromyces sambuceus and Cladosporium suaveolens efficiently generate coconutflavoured lactones γ-decalactone and δ-dodeca-lactone from ricinoleic acid and
linoleic acid, respectively (Kapfer et al. 1989; Allegrone et al. 1991). The yield of
lactones carried by yeasts via de novo fermentation is low, which can be increased
by supplying a limiting intermediate or precursor molecule to the fungal culture.
142
T. Malik and S. Rawat